Static analysis fix: let clang know _this->displays isn't NULL.

src/video/SDL_video.c

@@ -115,6 +115,7 @@ static SDL_VideoDevice *_this = NULL;
         SDL_UninitializedVideo(); \
         return retval; \
     } \
+    SDL_assert(_this->displays != NULL); \
     if (displayIndex < 0 || displayIndex >= _this->num_displays) { \
         SDL_SetError("displayIndex must be in the range 0 - %d", \
                      _this->num_displays - 1); \